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Evaluation of Treatment Planning System Dose Calculation in Irregular Rectangular Small Fields

A Qureshi

A Qureshi*, N Sperling , University of Toledo Medical Center, Toledo, OH


SU-I-GPD-T-586 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To evaluate the accuracy of dose computation in small field dosimetry as performed by modern treatment planning systems (TPS) compared to measured outputs.

Methods: The Exradin W1 Plastic Scintillator (Standard Imaging) was used to measure output factors taken on the Varian Edge and compared to output factors calculated using the Raystation and Pinnacle TPS. Measurements were taken with the W1PSD in the horizontal orientation, under 6MV and 6FFF beams at 10cm depth, 100cm SSD in a 48x48x41cm³ IBA BluePhantom2 water tank with both jaws independently varied from 0.6cm to 3.0cm. Output factors for the TPSs were calculated under similar conditions with a 0.1x0.1x0.1cm³ dose grid, using output factors measured with the W1PSD for their respective output factor calculations. Output Factors were measured as:OF_FS = M_FS/M_RefWhere M_FS and M_Ref are the measured signals for the clinical and reference fields (5x5cm²). Output Factors, measured and calculated, were then analyzed to evaluate how each TPS handles irregular rectangular small fields.

Results: Unlike Raystation, Pinnacle is unable to extrapolate below the smallest field size for which commissioning data was available, as such the smallest calculatable field in Pinnacle was 1x1cm². Compared to the W1PSD, OF calculated at a 1x1cm² field size with Raystation varied between +0.17% to +1.7% at 6MV and -2.3% to +0.62% at 6FFF, and Pinnacle varied between –7.1% to +1.14% at 6MV and -0.71% to +2.6% at 6FFF. In Raystation, fields smaller than the smallest commissioned size are extrapolated, resulting in large variations in the computed OF as much as -15.9% at 6MV and -21% at 6FFF for a 0.6x0.6cm² field.

Conclusion: Both TPS handle small field dose calculation differently. Raystation and Pinnacle exhibit large variability in the calculation of dose in fields traditionally considered small, converging on the measured outputs of the W1PSD at larger field sizes.

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